1. Field of the Invention
This invention pertains to electrical devices including switches and capacitors, and more particularly to a gradually-actuating device which may be used to form switches and/or variable-valued circuit elements.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Microelectromechanical switches, or switches made using microelectro-mechanical systems (MEMS) technology, are of interest in part because of their potential for allowing integration of high-quality switches with circuits formed using integrated circuit (IC) technology. As compared to transistor switches formed with conventional IC technology, for example, MEMS switches may exhibit lower losses and a higher ratio of off-impedance to on-impedance. A persistent problem with implementation of MEMS switches has been the high voltage required (often about 10V or higher) to actuate the switches, as compared to typical IC operating voltages (about 5V or lower).
These relatively high actuation voltages of MEMS switches are caused at least in part by a tradeoff between the closing and opening effectiveness of a given switch design. In the case of a cantilever switch, for example, approaches to lowering the actuation voltage of the switch include reducing the stiffness of the cantilever beam and reducing the gap between the contact element on the beam and the underlying contact pad. Unfortunately, these design changes typically have the effect of making opening of the switch more difficult. MEMS cantilever switch designs generally use an applied voltage to close the switch, and rely on the spring force in the beam to open the switch when the applied voltage is removed. In opening the switch, the spring force, or restoring force, of the beam must typically counteract what is often called xe2x80x9cstictionxe2x80x9d. Stiction refers to various forces tending to make two surfaces stick together, such as van der Waals forces, surface tension caused by moisture between the surfaces, and/or bonding between the surfaces (e.g., through oxidation). In general, modifications to a switch which act to lower the closing voltage also tend to make the switch harder to open, such that efforts to form a switch with a lowered closing voltage can result in a switch which may not open reliably (or at all). It would therefore be desirable to develop a switch design which relaxes the constraints imposed by the above-described tradeoff between opening and closing effectiveness.
The problems outlined above may be in large part addressed by a method for forming a micromechanical device in which a force associated with operation of the device is varied between locations spaced across a conductive element of the device. The variation in one or more forces across the conductive element of the device may advantageously give rise to a xe2x80x9crollingxe2x80x9d motion when the conductive element is brought toward the conductive pad, such that one part of the conductive element comes into the proximity of the pad before other parts do. Such a motion may in some embodiments allow a lower applied force to be used in bringing the conductive element toward the conductive pad than is needed to move a conductive element of similar area which moves xe2x80x9call at once.xe2x80x9d Alternatively or in addition, the force variation may give rise to a xe2x80x9cpeelingxe2x80x9d motion when the conductive element moves away from the conductive pad, in which one part of the conductive element moves away from the conductive pad before other parts do. This motion may in some embodiments reduce the tendency for the conductive element to become xe2x80x9cstuckxe2x80x9d in the vicinity of the conductive pad. In some embodiments, the device may be designed such that stable intermediate configurations are obtained in which only a portion of the conductive element is in the vicinity of the conductive pad. Such an embodiment may be used in forming a variable-valued circuit element, as discussed further below.
In a preferred embodiment of the method for forming a micromechanical device, the conductive element is attached to an actuating member of the device, and the variation of the force is in a direction not parallel to the longitudinal axis of the actuating member. The conductive element may in some embodiments be integral to or a part of the actuating member. In an embodiment, the force is a required force for movement of the conductive element toward a conductive pad positioned opposite the conductive element. The conductive element may make contact with the conductive pad during operation of the device, or there may be an insulator between the conductive pad and conductive element, such that they form plates of a capacitor. The force which is varied may also include a force applied to the conductive element during operation of the device, a restoring force tending to pull the conductive element away from a conductive pad, and/or a sticking force between the conductive element and the pad.
In an embodiment, the method may include patterning a first conductive layer arranged over a substrate to form a conductive pad, and patterning a second conductive layer arranged over the first conductive layer to form a conductive element. The patterning of the first conductive layer may include shaping the conductive pad to provide at least a portion of the variation in force across the conductive element. Patterning of the first conductive layer may also form a control element adapted for inducing movement of the conductive element toward the conductive pad. In such an embodiment, the patterning may include shaping the control element to provide at least a portion of the variation in force. Patterning of the second conductive layer may include shaping the conductive element to provide at least a portion of the variation in force. In an embodiment, patterning of the second conductive layer includes forming the actuating member, such as a cantilever arm, containing the conductive element, and shaping the member to provide at least a portion of the variation in force. The member may be shaped in various ways, including by forming openings within the member, where the density of the openings may vary in a direction transverse to the member, or in another direction across the member.
The method may further include forming a sacrificial layer over the first conductive layer and forming the second conductive layer over the sacrificial layer, before patterning the second conductive layer. The sacrificial layer may then be removed after patterning of the second conductive layer. In an embodiment, the upper surface of the sacrificial layer may be contoured before formation of the second conductive layer. Such contouring may allow shaping of a contacting portion of the subsequently-formed conductive element, and the shaping may provide at least a portion of the variation in force across the conductive element.
A method such as that described above may be used to form a switch contemplated herein. The switch is adapted such that a force associated with actuation of the switch varies between locations spaced across a contact element of the switch. In a preferred embodiment, the contact element is attached to an actuating member of the switch, and the variation of the force is in a direction not parallel to the longitudinal axis of the actuating member. The force may include, for example, a required closing force for the switch, an applied force during actuation of the switch, a restoring force tending to open the switch, and/or a sticking force tending to keep the switch closed. The force may in some cases vary monotonically from one side of the contact element to an opposing side of the contact element. In an embodiment, the switch is a cantilever switch, and the force varies in a direction transverse to the arm of the cantilever.
An embodiment of the switch may include the contact element, a contact pad adapted to make electrical contact with at least a portion of the contact element upon closing of the switch, and a control element for inducing movement of the contact element toward the contact pad. The shapes of one or more of these parts of the switch may be adapted to provide at least a portion of the variation in force across the contact element. Such shape adaptation may include asymmetric shapes and/or shapes having openings formed within them.
In addition to the switch described above, a variable-valued circuit element is contemplated herein. The circuit element may include a conductive element, a conductive pad, and a control element for inducing movement of the conductive element toward the conductive pad. The circuit element may be adapted such that a fraction of the conductive element which is moved to the proximity of the conductive pad is variable, depending on a total magnitude of a force applied using the control element. In an embodiment for which an insulator is interposed between the conductive pad and the conductive element, the circuit element may be used as a variable capacitance. In another embodiment, the conductive pad may be divided into multiple separate portions, where the number of portions contacted by the conductive element depends on the force applied using the control element. Each of these portions of the conductive pad may be connected to a respective fixed-value circuit element, so that a variable number of the fixed-value elements may be coupled to the conductive element. The fixed-value circuit elements may include, for example, capacitors, resistors and/or inductors. Each fixed-value circuit element may be connected between its respective conductive pad portion and a terminal common to all of the fixed-value circuit elements, such that those fixed-value elements being coupled to the conductive element are connected in parallel to one another.
In a manner similar to that for the switch and the method discussed above, the variable-valued circuit element may be adapted such that a quantity associated with motion of the conductive element is varied between locations spaced across the conductive element. In a preferred embodiment, the conductive element is attached to an actuating member of the circuit element, and the variation of the quantity is in a direction not parallel to the longitudinal axis of the actuating member. The quantity may include, for example, a force required to induce movement of the conductive element toward the conductive pad, a force applied using the control element, a restoring force tending to pull the conductive element away from the conductive pad, and/or a sticking force tending to keep the conductive element in contact with the conductive pad. The shapes of one or more parts of the circuit element may be adapted to provide some or all of the variation of the quantity.